text: Singlet fluorenylidene reacts with olefins in a concerted fashion, maintaining the stereochemistry of the reactant olefin. Triplet quenchers such as butadiene solvents can be used to increase stereospecific yields. Halogenated solvents also stabilize the singlet state. For example, dibromomethane and hexafluorobenzene deactivate the higher-energy singlet state, decelerating the rate of intersystem crossing in accordance with earlier studies of diphenylcarbene. The mechanism of singlet state deactivation is theorized to occur through halogen-lone pair complexation of empty 1Fl P-orbitals.